"unpack" std::array<T,N> as arguments to function - c++

Question

Here is quite nice (not mine) example how u can expand (or "explode") tuple as arguments to function:
template<int ...I> struct index_tuple_type {
template<int N> using append = index_tuple_type<I..., N>;
};
template<int N> struct make_index_impl {
using type = typename make_index_impl<N-1>::type::template append<N-1>;
};
template<> struct make_index_impl<0> { using type = index_tuple_type<>; };
template<int N> using index_tuple = typename make_index_impl<N>::type;
template <typename I, typename ...Args>
struct func_traits;
template <typename R, int ...I, typename ...Args>
struct func_traits<R, index_tuple_type<I...>, Args...> {
template <typename TT, typename FT>
static inline R call(TT &&t, FT &&f) {
return f(std::get<I>(std::forward<TT>(t))...);
}
};
template<
typename FT,
typename ...Args,
typename R = typename std::result_of<FT(Args&&...)>::type
>
inline R explode(std::tuple<Args...>& t, FT &&f) {
return func_traits<R, index_tuple<sizeof...(Args)>, Args...>
::call(t, std::forward<FT>(f));
}
then you can use this like so:
void test1(int i, char c) {
printf("%d %c\n", i, c);
}
int main() {
std::tuple<int, char> t1{57, 'a'};
explode(t1, test1);
}
Live version
I was wandering how could you do the same thing with std::array since it quite like tuple. std::get<N> works with std::array so I thought that it would be easy to modify this solution. But something like this doesn't work:
template<
typename FT,
typename Arg,
std::size_t I,
typename R = typename std::result_of<FT(Arg&&)>::type
>
inline R explode(std::array<Arg, I>& t, FT &&f) {
return func_traits<R, index_tuple<I>, Arg>::
call(t, std::forward<FT>(f));
}
void test2(int i1, int i2) {
printf("%d %d\n", i1, i2);
}
int main() {
std::array<int, int> t1{1, 2};
explode(t2, test1);
}
because of the part std::result_of<FT(Arg&&)>::type. The argument type Arg&& is wrong and result_of has no field type. For tuple Args&&... expanded, but now it should be "repeated" I times. Is there a way to do this using result_of so the returned type can be deducted?
Also i was wondering, having the tools to "unpack" tuple and array would it be possible to "unpack" recursively (probably using enable_if) structure like tuple<array<int, 2>, tuple<array<double,3>, ... and so on? Some kind of a tree where tuple and array are branches, and other types are leaves?

Answer 1

// enable argument dependent lookup on `get` call:
namespace aux {
using std::get;
template<size_t N, class T>
auto adl_get( T&& )->decltype( get<N>(std::declval<T>()) );
}
using aux::adl_get;
template<class F, class TupleLike, size_t...Is>
auto explode( F&& f, TupleLike&& tup, std::index_sequence<Is...> )
-> std::result_of_t< F( decltype(adl_get<Is>(std::forward<TupleLike>(tup)))... ) >
{
using std::get; // ADL support
return std::forward<F>(f)( get<Is>(std::forward<TupleLike>(tup))... );
}
is the first step. std::index_sequence is C++14, but it is easy to implement in C++11.
The next steps are also easy.
First, a traits class that dictates what types are tuple-like. I would go ahead and just duck-type use them, but a number of functions and traits classes we are going to use are not SFINAE friendly:
template<class T>
struct tuple_like:std::false_type{};
template<class... Ts>
struct tuple_like<std::tuple<Ts...>>:std::true_type{};
template<class... Ts>
struct tuple_like<std::pair<Ts...>>:std::true_type{};
template<class T, size_t N>
struct tuple_like<std::array<T,N>>:std::true_type{};
Next, an overload of explode that only works on tuple_like types:
template<class F, class TupleLike,
class TupleType=std::decay_t<TupleLike>, // helper type
class=std::enable_if_t<tuple_like<TupleType>{}>> // SFINAE tuple_like test
auto explode( F&& f, TupleLike&& tup )
-> decltype(
explode(
std::declval<F>(),
std::declval<TupleLike>(),
std::make_index_sequence<std::tuple_size<TupleType>{}>{}
)
)
{
using indexes = std::make_index_sequence<std::tuple_size<TupleType>{}>;
return explode(
std::forward<F>(f),
std::forward<TupleLike>(tup),
indexes{}
);
}
If you lack constexpr support you need to change some {} to ::value .
The above does the trick for pairs, arrays or tuples. If you want to add support for other tuple-like types, simply add a specialization to tuple_like and ensure std::tuple_size is specialized properly for your type and get<N> is ADL-overloaded (in the type's enclosing namespace).
std::make_index_sequence is also C++14 but easy to write in C++11.
template<size_t...>
struct index_sequence{};
namespace details {
template<size_t count, size_t...Is>
struct mis_helper:mis_helper<count-1, count-1, Is...> {};
template<size_t...Is>
struct mis_helper<0,Is...> {
using type=index_sequence<Is...>;
};
}
template<size_t count>
using make_index_sequence=typename details::mis_helper<count>::type;
(this is poor QOI for a C++14 library, which should use at least log descent, as it requires O(n) template recursive template instantiations for a list of size n. However, is n is less than a few 100, it won't matter).
std::enable_if_t<?> is C++14, but in C++11 is just typename std::enable_if<?>::type.